The present invention relates to desalting/concentrating apparatus by means of a reverse osmosis membrane. The apparatus pressurizes feed fluid under high pressure and supplies it to the membrane, and separates the fluid into a permeate fraction which permeates through the membrane and a concentrate fraction which does not permeate it.
U.S. Pat. No. 4,432,876 to Keefer shows a prior art reverse osmosis apparatus. With particular reference to FIG. 2 of the patent, a reciprocating feed pump 60 includes a pumping chamber 26.2 and an expansion chamber 37.2 which contains a portion of a piston rod 63 and is therefore smaller in cross-sectional area than the pumping chamber 26.2. The pumping chamber 26.2 connects through check valves 32.2 and 33.2 respectively with a feed fluid conduit 28.2 and a membrane pressure vessel 17 (FIG. 1 of the patent). The expansion chamber 37.2 connects through directional valves 42.2 and 44.2 respectively with the membrane pressure vessel and an exhaust conduit.
A pumping stroke forces the feed fluid out of the pumping chamber 26.2, through the membrane pressure vessel and into the expansion chamber 37.2 under high pressure because of the difference in volume between these chambers, thereby separating a permeate fluid fraction 13.2. An intake or induction stroke sucks a feed fluid into the pumping chamber 26.2, and exhausts a concentrate fluid fraction 14.2.
The directional valves 42.2 and 44.2 are timed in association with the reciprocating feed pump 60 or the reciprocable drive 48 (FIG. 1), via an electric control circuit or an interlocking mechanism, to open after initiation of a pumping or exhaust stroke respectively so that the valve actuation lags behind the piston stroke.
Upon initiation of a pumping or induction stroke, there is a sudden pressure rise respectively in the pumping or expansion chamber as the appropriate directional valve has not yet opened. This pressure rise is reduced by a volume exchange means 64 provided between the outfeed and return conduits 30.2 and 38.2.
The provision of the electrical control circuit or the interlocking mechanism and the volume exchanger 64 increases the cost, size and weight of the apparatus. In addition, the exchanger 64 interrupts the pump operation for an instant at each end of the pump stroke, thus reducing the concentration capacity.
It is an object of the present invention to provide a reverse osmosis apparatus without the above problems.